How Geothermal Energy Works
Heat from the Earth, called geothermal energy, heats water that has seeped
into underground reservoirs. These reservoirs can be tapped for a variety
of uses, depending on the temperature of the water. The energy from high
temperature reservoirs (225° to 600°F) can be used to produce
There are currently three types of geothermal power plants—dry steam,
flash steam, and binary cycle.
Dry steam power plants pipe steam from underground wells to the power
plant, where it is used to rotate a turbine, which activates a generator
to produce electricity. There are only two known underground resources of
steam in the United States: The Geysers in northern California and Old
Faithful in Yellowstone National Park. Since Yellowstone is protected from
development, the power plants at The Geysers are the only dry steam plants
in the country.
Flash steam power plants, which use waters at temperatures greater than
360°F, are the most common. As this hot water flows up through wells in
the ground, the decrease in pressure causes some of the water to boil into
steam. The steam is then used to power a generator, and any leftover water
and condensed steam is returned to the reservoir.
Binary cycle power plants use the heat from lower temperature
reservoirs (225° to 360°F) to boil a working fluid, which is then
vaporized in a heat exchanger and used to power a generator. The water,
which never comes into direct contact with the working fluid, is then
injected back into the ground to be reheated.
How Geothermal Energy is Used
Low to medium temperature waters (70° to 200°F) can be used directly to
heat buildings, grow and dry crops, melt snow on sidewalks, and for fish
and alligator farms. High temperature geothermal reservoirs (225° to 600°F)
are generally developed to produce large-scale electricity for
distribution by power providers. Our Buying
Clean Electricity section provides information on buying electricity
generated from geothermal energy and other renewable resources in your
Geothermal heat pumps allow home and business owners to take advantage
of the Earth's constant temperature (around 55°F) just a few yards
beneath the surface to heat and cool buildings, and to produce hot water.
Learn more about geothermal
Where Geothermal Energy is Used
Current drilling technology limits the development of geothermal resources
to relatively shallow, water- or steam-filled reservoirs, most of which
are found in the western part of the United States.
Researchers are now developing new technologies for capturing the heat
in deeper, "dry" rocks, which would support drilling almost
Geothermal heat pumps can be used in almost any part of the country.
The only real requirement is enough soil in which to bury the
heat-exchange pipes. Geothermal heat pumps have proved most popular in
areas with large heating requirements, such as the Northeast and the
northern Midwest, but they have been installed in almost every state in